1. Field of the Invention
The present invention relates to a coupler suitable for joining substrates with different thermal expansion coefficients, such as a semiconductor chip, and an organic printed circuit board, to construct a multi-layer structure.
2. Description of the Related Art
In a field of assembling constitutional components or elements of an electronic device, solder balls are generally used to connect a semiconductor chip (such as a silicon chip) with an organic printed circuit board to mount the semiconductor chip. The semiconductor chip has a thermal expansion coefficient of about 3 ppm/xc2x0 C., whereas the organic printed circuit board has a thermal expansion coefficient as large as 20 to 60 ppm/xc2x0 C. The difference in thermal expansion coefficients between the semiconductor chip and the organic printed circuit board induces stress on the electronic device with an increase in temperature. There are cases where the stress forms cracks on the solder balls used for connecting the semiconductor chip and then causes a conduction failure of the device.
Various methods have been proposed to relax the stress. For example, FIG. 15 shows a method of applying a resin 3 around solder balls 2 to reinforce the connection with a semiconductor chip 1. The solder balls 2 connect pads (electrodes) 4a and 4b located face to face. A copper wiring layer 6 is formed inside the organic printed circuit board 5 to make an electrical connection to the pad 4b. 
FIG. 16 shows a method of forming an underfill layer 7 made from thermosetting resin containing a ceramic filler, such as silica, between a semiconductor chip 1 and an organic printed circuit board 5. In this method, the entire surroundings of the solder balls 2 are covered with the resin, which reinforces the connection between the semiconductor chip 1 and the organic printed circuit board 5.
In these methods which use a resin as a reinforcing material, specific production equipment is required and a step of forming a resin layer, in addition to a step of connecting the solder balls, is necessarily employed, increasing the cost of manufacturing the device. Furthermore, in the method shown in FIG. 16, because the semiconductor chip 1 and the organic printed circuit board 5 are firmly adhered to each other and generate stress, there are cases 1) where the semiconductor chip 1 is broken, 2) where the organic printed circuit board 5 is warped and 3) where voids which are likely to be formed in the underfill layer 7 leading to cracking between the semiconductor chip 1 and the organic printed circuit board 5.
FIG. 17 shows a method of using wire bonds 8 which are formed to connect a semiconductor chip 1 and an organic printed circuit board 5 instead of the solder balls 8. Special production equipment is required to perform the process by this method and increased steps inevitably increase the cost of a device.
Japanese Patent Laid-Open No. Hei 5-259166 discloses a method of forming a structure on an electrode of a semiconductor chip, where metal dendrites made from copper, etc., grown on the electrode, are filled up with a resin. However, this method has such problems that (1) specific production equipment different from the conventional equipment is necessarily employed, making the production process complicated and increasing the production cost, (2) it is difficult to form the metal dendrites uniformly, and (3) filling the resin among the metal dendrites is likely to form voids to deteriorate the reliability of the semiconductor chip.
Japanese Patent Laid-Open No. Hei 5-275485 discloses a structure in which a metal layer has an H cross-section, the circumference of which is covered with a resin, so as to prevent the formation of bump cracks. However, it requires a lithographic process and a plating process for forming the metal layer and the resin layer, which are relatively expensive steps in manufacturing an electronic device.
Japanese Patent Laid-Open No. Hei 7-312400 discloses a method which uses a thin wire as an electrode of wire bonding to form junctions, wherein the thin wire includes a core made from a resin, a ceramic or a high melting point metal and a covering of the core made from a low melting point solder. In recent years, semiconductor chips have increased in the number of electrodes for input and output, and may reach as many as one thousand and more. The increase of electrodes in number will make it disadvantageous to form a solder layer for connecting by the wire bonding method from the standpoint of tact time of the process.
The invention has been made to solve the foregoing problems associated with the conventional methods, and an object of the invention is to achieve highly reliable parts assembling on the basis of the solder ball techniques which have been generally employed.
The present invention is to provide an electronic device assembled using a coupler which is capable of connecting an element carrying substrate and a wiring substrate with high reliability. According to the invention, there is provided an electronic device which comprises: an element carrying substrate having an electronic element and a first electrode, the first electrode disposed on a surface of the element carrying substrate and having a prescribed area; a wiring substrate arranged to face the element carrying substrate and having a second electrode disposed on a surface of the wiring substrate, the second electrode having a prescribed area and facing the first electrode; and a coupler disposed between the first electrode and the second electrode to join the element carrying substrate and the wiring substrate, the coupler having a resin body and an electroconductive member, the surface of the coupler comprising a resin region and an electroconductive region to thereby electrically connect the first electrode and the second electrode.
It is preferable that the resin region occupies from 20 to 80% of the surface of the coupler.
The coupler may be formed from a blend of the heat resistant resin and a joining metal to make a resin region and an electroconductive region, respectively.
The coupler may be formed by molding a blend of the heat resistant resin and metal powders, wherein metal powders locating on the surface of the coupler have a joining metal joined thereto.
The coupler may be formed from a heat resistant resin, wherein an encircling metal strip with a joined metal joined thereto is formed on the surface of the resin to make the electroconductive region. The encircling metal strip on the surface of the coupler may have an opening.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but not restrictive, of the invention.